(function(designWidth, maxWidth) {
    var doc = document,
      win = window,
      docEl = doc.documentElement,
      remStyle = document.createElement("style"),
      tid;
  
    function refreshRem() {
      var width = docEl.getBoundingClientRect().width;
      maxWidth = maxWidth || 540;
      width > maxWidth && (width = maxWidth);
      var rem = (width * 100) / designWidth;
      remStyle.innerHTML = "html{font-size:" + rem + "px;}";
    }
  
    if (docEl.firstElementChild) {
      docEl.firstElementChild.appendChild(remStyle);
    } else {
      var wrap = doc.createElement("div");
      wrap.appendChild(remStyle);
      doc.write(wrap.innerHTML);
      wrap = null;
    }
    //要等 wiewport 设置好后才能执行 refreshRem，不然 refreshRem 会执行2次；
    refreshRem();
  
    win.addEventListener(
      "resize",
      function() {
        clearTimeout(tid); //防止执行两次
        tid = setTimeout(refreshRem, 300);
      },
      false
    );
  
    win.addEventListener(
      "pageshow",
      function(e) {
        if (e.persisted) {
          // 浏览器后退的时候重新计算
          clearTimeout(tid);
          tid = setTimeout(refreshRem, 300);
        }
      },
      false
    );
  
    if (doc.readyState === "complete") {
      doc.body.style.fontSize = "16px";
    } else {
      doc.addEventListener(
        "DOMContentLoaded",
        function(e) {
          doc.body.style.fontSize = "16px";
        },
        false
      );
    }
  })(1920, 1920);
  